/Engine/Source/Editor/UnrealEd/Classes/MaterialGraph/MaterialGraph.h
Add an Expression to the Graph
UMaterialGraphNode* Newly created Graph node to represent expression
[UMaterialGraphNode](API\Editor\UnrealEd\MaterialGraph\UMaterialGraphNode) * AddExpression ( [UMaterialExpression](API\Runtime\Engine\Materials\UMaterialExpression) * Expression, bool bUserInvoked )